Remove use of deprecated behavior involving copy members

*  If any of the destructor, copy assignment or copy constructor
   are user-declared, both copy members should be user-declared,
   otherwise the compiler-generation of them is deprecated.

src/ripple/basics/CountedObject.h

@@ -112,6 +112,8 @@ public:
         getCounter ().increment ();
     }
 
+    CountedObject& operator=(CountedObject const&) = default;
+
     ~CountedObject ()
     {
         getCounter ().decrement ();